Method of bonding ceramic orthodontic appliances

ABSTRACT

A method of enhancing the bondability of ceramic orthodontic brackets or other dental attachments for improving the bonding strength between such attachments and the tooth engaging surface utilizing an intermediate bonding agent such as an acrylic resin. The method of the present invention results in the application of a silicious glass-like material coating of the bonding surface of the dental appliance. The silicious glass-like material provides three functions: (1) the proper surface chemistry for the use of organofunctional silanes that act as adhesion promoters or primers to enhance the bond between the adhesive and the orthodontic bracket; (2) a medium whereby second phase particulates can be adhered to the bracket to form irregular surface projections to enhance bonding; and (3) a surface that can be etched to yield macro and micro pitting to enhance the bond between the appliance and the intermediate bonding agent.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the art of ceramicorthodontic appliances and more specifically, to methods for providingthe ceramic material with a porous glassy coating of high mechanicalstrength to enhance the bonding of the ceramic material such aspolycrystalline alumina and sapphire to the tooth enamel with aninterlying acrylic resin material.

In orthodontic surgical procedures, orthodontic brackets are typicallyattached to teeth by adhesive bonding. The surface of the teeth is oftenacid etched so that a micro-mechanical interlock is achieved between theresin and the etched surface. Alternatively, the surface of the toothmay be provided with a crystal growth to which a resin material isbonded. There is also mechanical bond between the bracket base abuttingthe tooth and the resin. This mechanical bond normally represents theweak point of the assembly and brackets can become detached from theteeth by failure of the mechanical bond of the bracket resin interfaceif the shear strength thereof is exceeded. More recently, there has beena substantial interest developed in ceramic orthodontic appliancesbecause of the inherent wear resistance of ceramic materials. Inaddition, certain ceramic materials show substantial promise for use inmore cosmetic dental appliances wherein the dental brackets may be madefar less obvious by using transparent or translucent ceramic materialstherefor. In particular, it has been found that polycrystalline aluminaand sapphire can be used to create extremely wear-resistant orthodonticbracket appliances which also have superior cosmetic appearance andwhich may therefore have a profound effect on the orthodontic industryby making the wearing of orthodontic appliances more amenable to theappearance concerns of adults and older teenagers. Unfortunately, theaforementioned bonding problem of prior art orthodontic materials iseven more severe in regard to ceramic orthodontic appliances becausesuch ceramic materials inherently have extremely hard surfaces which aremore resistant to adherence to bonding materials.

2. Prior Art

Although there are numerous prior art references relating to the bondingof orthodontic appliances to the surface of teeth, the prior art withrespect to the bonding of ceramic materials for this purpose is quitelimited because of the relatively recent consideration of ceramicmaterials for orthodontic applications.

U.S. Pat. No. 3,895,445 to Silverman et al is directed to adhesivecompositions for orthodontic applications and specifically includes athermosetting resin composition which adheres to tooth enamel. The resincomposition is etched using a methacrylic acid ester mixture topenetrate the thermosetting resin composition and allow increasedadherence to the orthodontic appliances.

U.S. Pat. No. 3,955,282 to McNall is directed to a method of mountingorthodontic brackets to teeth. In this reference the tooth is etched byphosphoric acid and a pre-mixed dental adhesive attaches the orthodonticbrackets to the teeth.

U.S. Pat. No. 4,180,911 to Bullock is directed to a method for directbonding of orthodontic structures to teeth using a fluoridepre-treatment. The tooth is etched by acid and a solution of stannousfluoride is applied to the dried tooth surface to protect from thedevelopment of caries and also act as a bridging means between the toothsurface and the adhesive to enhance the bond strength.

U.S. Pat. No. 4,010,545 to Kilian et al is directed to adhesive bondingof orthodontic brackets to teeth. The tooth surface is acid etched,rinsed and dried. A thixotropic activated monomer is applied to thesurface of the tooth or to the orthodontic bracket. The thixotropicactivated monomer is activated prior to the application.

U.S. Pat. No. 3,625,916 to Newman is directed to a synthetic plasticdental adhesive. This reference is relevant because it relates to theuse of silica and a filler selected from the group consisting of silicagel and pulverulent, fused quartz glass.

U.S. Pat. No. 4,068,379 to Miller et al discloses an orthodonticappliance with a porous tooth-abutting face. The appliance is bondeddirectly to a tooth utilizing a composite base with a poroustooth-abutting face. The base is formed by rolling a mesh in strip formto form flats on at least one surface of the mesh which is to be incontact with a solid base portion and then bonding the mesh to the base.

Canadian Patent No. 1,187,727 to Smith et al relates to the use of aparticular coating applied to dental attachments on the tooth engagingsurface thereof which coating enables improved bonding between the toothand the dental attachment. Methods such as fusion or sintering may beused to apply the particular coating to the attachment. The coating maytake the form of discrete particles spaced apart from each other on thesurface of the form of a layer or multiple layers of particles bondedtogether to produce a network of interconnected pores.

Unfortunately, none of the prior art known to the applicant hereinspecifically addresses the problems associated with bonding an extremelyhard ceramic material orthodontic bracket to the surface of a tooth.Accordingly, none of the prior art known to the applicant teaches oreven suggests the particular solution of the present invention.

SUMMARY OF THE INVENTION

The present invention relates to a method of increasing the bondabilityof a ceramic material such as polycrystalline alumina or sapphire, to bebonded by means of an acrylic resin to the surface of tooth enamel.Basically, the method described herein produces a porous glassy coatingwhich matches the coefficient of thermal expansion of the ceramic suchas alumina or sapphire and engenders a wicking mechanism to enhance thebonding area of the acrylic resin. The siliceous glassy coating alsoprovides a chemically compatible surface to allow the use oforganofunctional silane adhesion promoters or primers, the use of whichgreatly enhances the bond strength of the appliance to the acrylic orother resin glue. Without the use of this primer the bond strength istoo low to facilitate the successful use of the orthodontic appliance.As a result of the present invention the direct application oforthodontic and other dental attachments made from such ceramicmaterials is greatly facilitated.

Three alternative embodiments are disclosed herein. In one suchembodiment a frit material such as powdered glass or glassy ceramic isapplied to a polycrystalline or single crystal surface at the base of aceramic dental bracket. The frit is then fired to a melt condition andcooled to a glass or glassy ceramic condition. It is then etched inhydrofluoric acid to increase the surface area as the silica content ofthe frit is removed at a higher rate than the ceramic content.

In a second embodiment the base of a dental bracket is coated with aglass or glassy ceramic with a frit containing between 10 and 30% byweight, 100 micron or larger particles of alumina. Such particles appearat the surface of the glass or glassy ceramic surface interface andgreatly increase the surface area of the bond interface.

In still a third alternative embodiment of the present invention thealumina particles of the second embodiment of the invention are replacedby silica particles of the same size, namely, 100 microns or larger.After fritting, the large particles of silica are leached by immersionin hydrofluoric acid to produce a highly porous surface at the base ofthe dental bracket.

The organofunctional silane is applied to the etched surface by methodscommon to the bonding industry such as dipping, spraying or painting.After silanation the parts are dried and ready for bonding. It has beenfound that whereas unsilanated polycrystalline orthodontic brackets havea bond pull strength in the range of 2-6 pounds, silanatedpolycrystalline orthodontic brackets have a bond pull strength in therange of 38-62 pounds.

OBJECTS OF THE INVENTION

It is therefore a principal object of the present invention to provide amethod for enhancing the bonding of ceramic material orthodonticappliances to the surface of teeth using acrylic resins or other glues.

It is an additional object of the present invention to provide a methodfor increasing the bondability of polycrystalline alumina and/orsapphire substrates in the form of orthodontic appliances whereby topermit exploitation of the structural and cosmetic advantagesous use ofceramic materials for orthodontic applications.

It is still an additional object of the present invention to provide amethod for applying a glass or glassy ceramic material to the contactsurface of a dental bracket and including a frit material such aspowdered glass or powdered glassy ceramic which may be etched in aselected acid to increase the surface area of the contact portion of theceramic bracket in order to improve the bonding between the bracket andthe tooth.

It is still an additional object of the present invention to provide theincreased surface area of a ceramic dental bracket as described in theabove disclosed object of the invention, but by utilizing a fritcontaining a preselected quantity by weight of a minimum size ofparticles of either alumina or silica for increasing the surface area ofthe dental bracket contact portion.

It is still an additional object of the present invention to provide asiliceous surface to which an organofunctional silane adhesion promoteror primer bonds and enhances the adhesion between the orthodonticappliance and the acrylic resin or other glues.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned objects and advantages of the present invention aswell as additional objects and advantages thereof will be more fullyunderstood hereinafter as a result of a detailed description ofpreferred embodiments of the invention when taken in conjunction withthe accompanying drawing which represents an orthodontic appliancetreated in accordance with the methods of the present invention andindicating its relative position after being bonded to the enamelsurface of a tooth.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The method of the present invention may be best understood by firstreferring to FIG. 1 wherein a dental bracket or comparable orthodonticappliance 10 is illustrated and comprises a ceramic appliance body 12having a bonding surface 13 which is adapted by means of the presentinvention to be bonded to a tooth 18 by an intermediate bonding agent 15such as an acrylic resin. Normally the bonding of a hard ceramic surfacesuch as sapphire or crystalline alumina directly to an acrylic resinproduces a weak joint at best. However, increasing the surface area ofthe bonding region on the ceramic orthodontic appliance increases thepull strength of the bond. The method of the present invention is usedto significantly increase the strength of the bond between the bracket12 and the acrylic resin 15 along the surface 13 by applying a coatingof glass (silica) which substantially matches the coefficient ofexpansion of the ceramic material but more importantly by significantlyincreasing the surface area of the glass coating along surface 13 toovercome the otherwise inherently weak joint between the ceramic and theacrylic.

Three alternative embodiments of the method of the present invention aredisclosed herein to increase the bondability of the ceramic bracket tothe acrylic resin and thus to the tooth. As a result, the directapplication of orthodontic and other dental attachments made fromselected ceramic materials is greatly facilitated. The first embodimentof the present invention comprises the steps of applying the glass orglassy ceramic material, (generically referred to herein as glass-likematerial), which has a high silica content and which in fact comprises afrit material incluidng powdered glass or powdered glassy ceramic. Thisfrit material is applied to the surface 13 of the bracket 12 which maybe way of example comprise polycrystalline alumina or sapphire, both ofwhich have proved to be cosmetically appealing as well as significantlywear resistant for use in orthodontic appliances. After the fritmaterial is applied to the surface 13 the frit is fired, that is, heatedto a melt temperature and then allowed to cool. The surface 13 havingthe frit coating thereon is then etched in an appropriate acid such ashydrofluoric acid. The silica content of the glass-like material coatingon the surface 13 will be removed in the acid at a higher rate than theceramic content of the frit and as a result the coating applied tosurface 13 will become pitted depending upon the length of time of theetching. The pitting of course increases the total surface area of thesurface 13 and the combined wicking action of the porous glassy coatingas well as the increased surface area caused by the etching action ofthe hydrofluoric acid on the silica content of the coating,significantly enhance the strength of the bond between the bracket 12and the acrylic 15 along the surface 13. The bonding is further enhancedand facilitated by applying an organofunctional silane adhesion promoterto the etched high surface area of the siliceous frit coating. Standardtechniques as designated by manufacturers of these materials such as DowCorning or Union Carbide can be used to paint, dip or spray the silanecoupling agents onto the surface.

In an alternative embodiment of the present invention the glass-likematerial frit contains between 10 and 30% by weight particles of aluminawhich are preferably at least 100 microns in size. Such particles appearat the surface of the glass-like ceramic surface interface and therebygreatly increase the surface area of the bond interface along surface13.

In yet another embodiment of the present invention instead of usingalumina particles to enhance the bonding surface area, one utilizessilica particles that are also at least 100 microns in size. After thefrit coating is applied to the tooth the large silica particles are thenimmersed in an etching acid such as hydrofluoric to produce a highlyporous surface at the base of the dental bracket 12. The secondembodiment produces an increased surface area by, in effect, creating aglassy coating along the surface 13 that has a plurality of relativelylarge irregular projections. On the other hand, the third embodimentresults in an increased surface area wherein the applied glassy coatinghas a plurality of irregularly surfaced depressions or dimples resultingfrom the removal of the silica by the etching acid.

Those having skill in the art to which the present invention pertainswill now as a result of the teaching herein understand that the presentinvention pertains to a method for increasing the bond strength betweena ceramic and acrylic resin and more specifically to a method forincreasing the bond strength between a ceramic orthodontic appliance andthe surface of a tooth where, for example, an acrylic resin is employedas a bonding agent. Three alternative embodiments have been disclosedherein each directed toward achieving a substantially identical result,namely, the application of glassy coating to the bonding surface of theceramic dental appliance having a significantly enhanced surface areadue to the etching of silica particles or the retention of aluminaparticles as projections of the bonding surface. It will be furtherunderstood that various modifications and additions may be made to thepresent invention. By way of example, although specific ceramicmaterials, coating materials and surface area generating materials havebeen disclosed herein, it will be understood that various alternativematerials may also be employed without departing from the teaching ofthe invention. Accordingly, the scope of the present invention is to belimited only by the claims appended hereto.

I claim:
 1. A method for increasing the bond strength between a ceramicand a bonding agent such as acrylic resin, the method comprising thesteps of:applying a glass-like material coating to the ceramic, saidglass-like material coating having particles of powdered glass-likematerial therein and having a pre-selected content of silica in saidparticles; firing said glass-like material coating to a temperaturesufficient to melt said particles; cooling said glass-like materialcoating to harden same; and etching said glass-like material coating toremove said silica whereby to increase the surface area of said coatingfor increasing the area for receiving said acrylic resin; and silanatingsaid etched glass-like material coating with an organofunctional silaneadhesion promoter.
 2. The method recited in claim 1 wherein said ceramicis taken from the group consisting of polycrystalline alumina andsapphire.
 3. The method recited in claim 2 wherein said ceramic is in asingle crystalline form.
 4. The method recited in claim 2 wherein saidceramic is in a polycrystalline form.
 5. The method recited in claim 1wherein said particles comprise entirely silica and are at least 100microns in size.
 6. The method recited in claim 1 wherein said ceramicis formed into the shape of an orthodontic appliance and said bondingagent is used as a bonding interface between said appliance and thesurface of a tooth.
 7. The method recited in claim 1 wherein saidglass-like material is taken from the group consisting of glass andglassy ceramics.
 8. The method recited in claim 1 wherein said acid ishydrofluoric acid.
 9. A method for increasing the bond strength betweena ceramic orthodontic appliance and the surface of a tooth where anacrylic resin is employed as the bonding agent; the method comprisingthe step of coating the bonding surface of said appliance with aglass-like frit containing between 10 and 30% by weight particles ofalumina of at least 100 microns for increasing the surface area of saidbonding surface.
 10. The method recited in claim 9 wherein said ceramicis taken from the group consisting of alumina and sapphire.
 11. A methodfor increasing the bond strength between a ceramic and a bonding agentsuch as acrylic resin, the method comprising the steps of:melting aglass-like material coating on the ceramic and allowing said coating tocool until it is in a hardened state; and silanating said coating withan organofunctional silane adhesion promoter.
 12. The method recited inclaim 11 wherein said ceramic is polycrystalline alumina.
 13. A methodfor increasing the bond strength between a ceramic and a bonding agentsuch as acrylic resin; the method comprising the steps of:applying aglass-lke material coating to the ceramic; firing said glass-likematerial coating until it melts; cooling said glass-like materialcoating to harden same; and silanating said glass-like material coatingwith an organofunctional silane adhesion promoter.
 14. The methodrecited in claim 13 wherein said ceramic is polycrystalline alumina.